The following discussion is intended to facilitate the understanding of the invention, but is not intended nor admitted to be prior art to the invention.
A. Obesity and Diabetes
Obesity is the most common metabolic disease in developed nations. Despite public health education and initiatives, its prevalence continues to rise, with greater than 30% of adults in the United States being obese and greater than 60% of adults being overweight or obese. The World Health Organization has estimated that worldwide, over one billion adults are overweight, with at least 300 million of them being obese. Obesity leads to, or significantly increases the risk of, a wide range of comorbidities that includes but is not limited to hypertension, congestive cardiomyopathy, coronary heart disease, stroke, dyslipidemia, metabolic syndrome, and Type 2 diabetes [Bays, Obesity Research (2004) 12:11971211] and premature death. There is an unmet medical need for safe and effective antiobesity drugs as a therapeutic option with which to reduce the worldwide obesity epidemic.
The incidence of Type 2 diabetes in the United States is about 7% and accounts for as much as 10% of all health care dollars. Furthermore, the incidence of Type 2 diabetes worldwide is increasing such that Type 2 diabetes is now considered to be a worldwide epidemic. Type 2 diabetes is characterized by fasting and postprandial hyperglycemia and by relative insulin insufficiency. Hyperglycemia may cause long-term microvascular and macrovascular complications, such as nephropathy, neuropathy, retinopathy, and peripheral vascular disease. In addition, Type 2 diabetes is a comorbid disease that frequently compounds hyperlipidemia, atherosclerosis and hypertension. Hyperlipidemia is a primary risk factor for cardiovascular disease due to atherosclerosis. Type 2 diabetes causes significant morbidity and mortality at considerable expense to patients, their families and society.
B. Glucagon-Like Peptide-1 (GLP-1)
Glucagon-like peptide-1 (GLP-1) is an incretin hormone derived from the posttranslational modification of proglucagon and secreted by gut endocrine cells. GLP-1 mediates its actions through a specific G protein-coupled receptor (GPCR), namely GLP-1R. GLP-1 is best characterized as a hormone that regulates glucose homeostasis. GLP-1 has been shown to stimulate glucose-dependent insulin secretion and to increase pancreatic beta cell mass. GLP-1 has also been shown to reduce the rate of gastric emptying and to promote satiety. The efficacy of GLP-1 peptide agonists in controlling blood glucose in Type 2 diabetics has been demonstrated in several clinical studies [see, e.g., Nauck et al., Drug News Perspect (2003) 16:413-422], as has its efficacy in reducing body mass [Zander et al., Lancet (2002) 359:824-830].
GLP-1 receptor agonists are additionally useful in protecting against myocardial infarction and against cognitive and neurodegenerative disorders. GLP-1 has been shown to be cardioprotective in a rat model of myocardial infarction [Bose et al., Diabetes (2005) 54:146-151], and GLP-1R has been shown in rodent models to be involved in learning and neuroprotection [During et al., Nat Med (2003) 9:1173-1179; and Greig et al., Ann N Y Acad Sci (2004) 1035:290-315].
Certain disorders such as Type 2 diabetes are characterized by a deficiency in GLP-1 [see, e.g., Nauck et al., Diabetes (2004) 53 Suppl 3:S190-196].
Current GLP-1 peptide agonists suffer from a lack of oral bioavailability, negatively impacting patient compliance. Efforts to develop orally bioavailable non-peptidergic, small-molecule agonists of GLP-LR have so far been unsuccessful [Mentlein, Expert Opin Investig Drugs (2005) 14:57-64]. An attractive alternative approach is to develop an orally active composition for increasing an endogenous level of GLP-1 in the blood.
C. BRS-3
Bombesin is a 14 amino acid peptide isolated from frog skin. Bombesin Receptor Subtype-3 BRS-3 G protein-coupled receptor (BRS-3; e.g., human BRS-3, GenBank® Accession No. AAA35604 and alleles thereof; e.g., mouse BRS-3, GenBank® Accession No. AY288423 and alleles thereof) exhibits about 50% homology to gastric-releasing peptide receptor (GRP-R) and neuromedin B receptor (NMB-R), and together they form the bombesin-like receptor group. BRS-3 is selectively expressed in tissues including hypothalamus and uterus. BRS-3 activation leads to increased accumulation of intracellular inositol 1,4,5-triphosphate (IP3), consistent with BRS-3 being coupled to Gq. In recent studies, BRS-3 knockout mice developed obesity, diabetes, and hypertension [Ohki-Hamazaki et al., Nature (1997) 390:165-169].
D. Dipeptidyl Peptidase IV (DPP-IV)
Dipeptidyl peptidase IV (DPP-IV, EC 3.4.14.5) exhibits catalytic activity against a broad range of peptide substrates that includes peptide hormones, neuropeptides, and chemokines. The incretins glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), which stimulate glucose-dependent insulin secretion and otherwise promote blood glucose homeostasis, are rapidly cleaved by DPP-IV at the position 2 alanine leading to inactivation of their biological activity. Both pharmacological and genetic attenuation of DPP-IV activity is associated with enhanced incretin action, increased insulin, and lower blood glucose in vivo. Genetic attenuation of DPP-IV activity has been shown to provide resistance to obesity and to improve insulin sensitivity. A second-generation DPP-IV inhibitor, LAF237 (Ahren et al., J Clin Endocrinol Metab (2004) 89:2078-2084; and Villhauer et al., J Med Chem (2003) 46:2774-2789; the disclosure of each of which is herein incorporated by reference in its entirety), is currently in phase 3 clinical trials for Type 2 diabetes and additional DPP-IV inhibitors are in clinical development.
Because the incretin hormones are not the only substrates for DPP-IV, there is concern that inhibition of the cleavage of other endogenous DPP-IV substrates may give rise to undesirable side effects [see, e.g., Chen et al, J Biol Regul Homeost Agents (2004) 18:47-54, the disclosure of which is herein incorporated by reference in its entirety]. It therefore would be advantageous to identify an activity promoting blood glucose homeostasis which is associated with substantially lower concentrations of DPP-IV inhibitor.
E. G Protein-Coupled Receptors
GPCRs share a common structural motif, having seven sequences of between 22 to 24 hydrophobic amino acids that form seven alpha helices, each of which spans the membrane (each span is identified by number, i.e., transmembrane-1 (TM-1), transmembrane-2 (TM-2), etc.). The transmembrane helices are joined by strands of amino acids between transmembrane-2 and transmembrane-3, transmembrane-4 and transmembrane-5, and transmembrane-6 and transmembrane-7 on the exterior, or “extracellular” side, of the cell membrane (these are referred to as “extracellular” regions 1, 2 and 3 (EC-1, EC-2 and EC-3), respectively). The transmembrane helices are also joined by strands of amino acids between transmembrane-1 and transmembrane-2, transmembrane-3 and transmembrane-4, and transmembrane-5 and transmembrane-6 on the interior, or “intracellular” side, of the cell membrane (these are referred to as “intracellular” regions 1, 2 and 3 (IC-1, IC-2 and IC-3), respectively). The “carboxy” (“C”) terminus of the receptor lies in the intracellular space within the cell, and the “amino” (“N”) terminus of the receptor lies in the extracellular space outside of the cell.
Generally, when an agonist binds to a G protein-coupled receptor (often referred to as “activation” of the receptor), there is a change in the conformation of the receptor that facilitates coupling between the intracellular region and an intracellular “G-protein.” It has been reported that GPCRs are “promiscuous” with respect to G proteins, i.e., that a GPCR can interact with more than one G protein. See, Kenakin, T., 43 Life Sciences 1095 (1988). Although other G proteins may exist, currently, Gq, Gs, Gi, Gz and Go are G proteins that have been identified. Ligand-activated GPCR coupling with the G-protein initiates a signaling cascade process (referred to as “signal transduction”). Under normal conditions, signal transduction ultimately results in cellular activation or cellular inhibition.
Gs stimulates the enzyme adenylyl cyclase. Gi (and Gz and Go), on the other hand, inhibit adenylyl cyclase. Adenylyl cyclase catalyzes the conversion of ATP to cAMP; thus, activated GPCRs that couple the Gs protein are associated with increased cellular levels of cAMP. On the other hand, activated GPCRs that couple Gi (or Gz, Go) protein are associated with decreased cellular levels of cAMP. See, generally, “Indirect Mechanisms of Synaptic Transmission,” Chpt. 8, From Neuron To Brain (3rd Ed.) Nichols, J. G. et al eds. Sinauer Associates, Inc. (1992). Thus, assays that detect cAMP can be utilized to determine if a candidate compound is, e.g., an agonist to a Gs-associated receptor (i.e., such a compound would increase the levels of cAMP). Gq and Go are associated with activation of the enzyme phospholipase C, which in turn hydrolyzes the phospholipid PIP2, releasing two intracellular messengers: diacylglycerol (DAG) and inositol 1,4,5-triphosphate (IP3). Increased accumulation of IP3 is associated with activation of Gq- and Go-associated receptors. See, generally, “Indirect Mechanisms of Synaptic Transmission,” Chpt. 8, From Neuron To Brain (3rd Ed.) Nichols, J. G. et al eds. Sinauer Associates, Inc. (1992). Assays that detect IP3 accumulation can be utilized to determine if a candidate compound is, e.g. an agonist to a Gq- or Go-associated receptor (i.e., such a compound would increase the levels of IP3). Assays that detect the level of intracellular free calcium can also be utilized to determine if a candidate compound is, e.g., an agonist to a Gq or Go-associated receptor (i.e., such a compound would increase the levels of intracellular free calcium).
See, e.g., Table A (“N/A”: “not applicable”).
TABLE AEffect on cAMP ProductionEffect on IP3 AccumulationEffect on cAMPEffect on IP3upon Activation of GPCRupon Activation of GPCRProductionAccumulationG(i.e., constitutive activation(i.e., constitutive activationupon contact withupon contact withproteinor agonist binding)or agonist binding)an Inverse Agonistan Inverse AgonistGsIncreaseN/ADecreaseN/AGiDecreaseN/AIncreaseN/AGzDecreaseN/AIncreaseN/AGoDecreaseIncreaseIncreaseDecreaseGqN/AIncreaseN/ADecrease
There are also promiscuous G proteins, which appear to couple several classes of GPCRs to the phospholipase C pathway, such as Gα15 or Gα16 [Offermanns & Simon, J Biol Chem (1995) 270:15175-80], or chimeric G proteins designed to couple a large number of different GPCRs to the same pathway, e.g. phospholipase C [Milligan & Rees, Trends in Pharmaceutical Sciences (1999) 20:118-24]. Assays that detect the level of intracellular free calcium can be utilized to determine if a candidate compound is, e.g., an agonist to a GPCR coupled to the phospholipase C pathway (i.e., such a compound would increase the levels of intracellular free calcium).
Under physiological conditions, GPCRs exist in the cell membrane in equilibrium between two different conformations: an “inactive” state and an “active” state. A receptor in an inactive state is unable to link to the intracellular signaling transduction pathway to initiate signal transduction leading to a biological response. Changing the receptor conformation to the active state allows linkage to the transduction pathway (via the G-protein) and produces a biological response.
A receptor may be stabilized in an active state by a ligand or a compound such as a drug. Recent discoveries, including but not exclusively limited to modifications to the amino acid sequence of the receptor, provide means other than ligands or drugs to promote and stabilize the receptor in the active state conformation. These means effectively stabilize the receptor in an active state by simulating the effect of a ligand binding to the receptor. Stabilization by such ligand-independent means is termed “constitutive receptor activation.” An endogenous receptor exhibiting activity in the absence of ligand is referred to as a constitutively active endogenous receptor.